We’ve known some things from the beginning of Robert Mueller’s appointment as Special Counsel.

We’ve known that he was, from the get-go, hopelessly compromised due to his long friendship with one the major witnesses in the investigation: His former protégé, Jim Comey. The idea that Mueller would attempt to investigate and possibly prosecute Trump for obstructing justice by way of firing Comey was an uncomfortable one in every conceivable way.

We’ve known, too, that he would be incapable of properly investigating the FBI, seeing as how he was once the director of that (formerly) hallowed institution and no doubt would rather see his own investigation come to a flaming end than to throw the Bureau under the bus. And since much of what needs to be investigated in this case (as we’re seeing so clearly these days) has to do with the FBI and the Justice Department and their disturbing anti-Trump biases, he’s the wrong man for the job.

And we’ve known that he outfitted his Special Counsel team with Democrat after Democrat, Hillary donor after Hillary donor, making it all but impossible that he would be able to conduct a fair, impartial, thorough investigation that looked just as deeply at the exculpatory evidence as the damning stuff.

But we didn’t know that his entire investigation was built on fruit of the poisoned tree.

Fruit of the poisoned tree is a legal term meaning that any evidence collected through illegal means can no longer be used against a defendant in court. Often, it’s used when discussing evidence gathered without a proper warrant. That’s not what happened here, however. Instead, it seems that Mueller’s team improperly got their hands on documents and emails from the Trump transition team thanks to a government agency that had no business handing them over.

In a letter to Congress this week, attorney Kory Langhofer of Trump for America said that Mueller obtained emails, laptops, and cellphones from the transition team by way of the General Services Administration.

“The materials produced by the G.S.A. to the special counsel’s office therefore included materials protected by the attorney-client privilege, the deliberative process privilege, and the presidential communications privilege,” Langhofer wrote.

Mueller’s team issued a statement denying any wrongdoing. “When we have obtained emails in the course of our ongoing criminal investigation, we have secured either the account owner’s consent or appropriate criminal process.”

We’ll see about that. If it turns out that Mueller has interviewed suspects and built indictment cases off of documents and devices he should not have had in his possession, it could be the thread that unravels his entire investigation. When all of this comes out in the wash, Trump may not have to fire the special counsel; the special counsel’s sloppy work will invalidate his investigation all by itself.